Verhalten von Nanosilber in Kläranlagen und dessen Einfluss auf die Nitrifikationsleistung in Belebtschlamm

Abstract

Background, aim and scope The application of nanosilver is increasing. Knowledge on the fate and behavior of nanosilver in wastewater and wastewater treatment plants is scarce. Studies under real world conditions are completely lacking. We studied (1) the impact of nanosilver on the nitrification of sewage sludge, (2) quantified the mass flow of nanosilver in a pilot-plant, and (3) verified the mass balance in a full-scale municipal wastewater treatment plant where nanosilver is introduced to the municipal plant by an indirect discharger. Materials and Methods The addition of four different nanosilver additives on ammonia oxidation in activated sludge has been studied in batch-reactors at two concentrations (1, 100 mg/L Ag) with two exposure times (2 h, 6 days). The pilot-plant treating 70 population equivalents of domestic wastewater is operated with a 12 day sludge age. Nanosilver was applied to the activated sludge tank within two sludge ages. The silver concentrations were measured in sludge and effluent samples during dosing and the following two sludge ages. The adsorption and speciation of silver particles has been analyzed using scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX). Influent, effluent and sludge were sampled on a fullscale plant (60 000 equivalent inhabitants) and analyzed for silver. Results Silver nitrate, metallic nanosilver, nano-scaled silver chloride and microcomposite silver did not show any effect on ammonia oxidation after the addition of 1 mg/L Ag to the activated sludge (corresponding to 250 mg Ag per kg solids). In contrast, 100 mg/L Ag inhibited the nitrification process by 100 % after the addition of silver nitrate and 20- 30 % after addition of colloidal polymer-coated nanosilver. A complete mass balance of the pilot-plant, a steady-state system with known fluxes, demonstrates significant enrichment of silver in the sewage sludge (96 %) after the addition of silver chloride to the plant and small losses of silver into the secondary effluent (4 %). The mass fluxes are similar to metallic colloidal nanosilver investigated under the same conditions. The mass balance has been consistently confirmed by the full-scale study in a municipal wastewater treatment plant. The silver fluxes correspond to the fluxes of the suspended solids in sludge and effluent. Overall, it is estimated that from public wastewater treatment plants about 4-40 mg/a Ag per inhabitant equivalent are discharged annually to the receiving water. The analysis by SEM-EDX demonstrates adsorption and incorporation of nanosilver on biological flocs. This method yields first insight into complex building and transformation of silver associated with sulfide after adding metallic nanosilver and silver chloride to wastewater. Discussion Silver ions released from nanosilver react immediately with large amounts of chloride present in wastewater to form silver chloride. Silver ions may react with organic ligands or sulfide groups additionally. Consequently, even silver nitrate added with 1 mg/L Ag (250 mg Ag/kg TS) to activated sludge did not inhibit nitrification activity. Very high amounts of nanosilver, i. e. 100 mg/L Ag, overburdened the system and equilibrium condition between silver ion release and ligands was not reached. The mass balance reflects the excellent attachment of nanosilver to activated sludge and biological flocs. Therefore, the main elimination process of nanosilver is attachment to the activated sludge. The elimination of nanosilver is high compared to organic and inorganic micro pollutants omnipresent in wastewater. Any further reduction of suspended solids in the effluent water will reduce the silver load. Conclusions Generally, in wastewater nanosilver occurs bonded to activated sludge flocs and therefore the elimination of nanosilver is efficient under operation conditions typical for wastewater treatment plants. The major fraction of nanosilver is removed from the system by the excess sludge withdrawal. Nonetheless, the efficiency may be further improved by a tertiary filtration step. All analyses of influent, effluent and sludge confirmed that silver exists as silver sulfide. Due to a negligible water solubility of this silver species, silver ions are not subsequently released. Recommendations and perspectives Nanosilver research should be driven to the identification of transformation processes in real environmental matrices and the influence of coatings on the adsorption behavior. There is no need for action to take special measures for nanosilver removal in the area of municipal wastewater treatment plants compared to ubiquitous hazardous organic pollutants may present in wastewater. Nevertheless, source control measures following the precautionary principle should be performed. © Springer-Verlag 2010.